For the purpose of maintaining a surrounding environment at a constant relative humidity, a sheet product containing a moisture adsorbent is used as a packing material during storage or transportation of an art object, an electric product, a craft product, clothes, etc., a house interior finishing material, a moisture absorption agent in a closet, etc. Further, a sheet product capable of moisture absorption and moisture release is also used in a dehumidifier in an air-conditioning apparatus or a dehumidifier device for dehumidifying and humidifying air in a room and a total heat exchanger device for ventilating a room while exchanging a temperature (heat) and a humidity (moisture) with each other between air exhaustion and air suction. In the air-conditioning apparatus or the total heat exchanger device, a laminate obtained by stacking corrugated sheet products or a product that is obtained by winding a sheet product in the form of a rotor is used as a dehumidifier device or heat exchanger device.
Conventionally, the moisture adsorbent is selected from organic moisture adsorbents such as a super absorbent polymer, carboxymethyl cellulose, etc., and inorganic moisture adsorbents such as sepiolite, zeolite, bentonite, attapulgite, diatomite, activated carbon, silica gel, aluminum hydroxide, etc. These moisture adsorbents absorb a large amount of moisture. However, they have a problem that they take a time to decrease a relative humidity due to the sluggish speed of moisture absorption. In particular, in an air-conditioning apparatus and a total heat exchanger device, it is required to perform dehumidification for a short period of time for which air is flowing, and it has hence been an important object to improve the speed of moisture absorption.
When a sheet product containing a water absorption agent is used as a packaging material, a water absorption agent in a closet, etc., it is required to make the moisture absorption agent release moisture by drying the sheet product by means of sunlight, etc., under an ordinary temperature environment when it is regenerated. When it is used in an air conditioning apparatus or total heat exchanger device, the sheet product is required to have a large speed of moisture release since it is required to repeat moisture absorption and moisture release for a short period of time or perform moisture permeation in the thickness direction. Since the above moisture adsorbents that have been hitherto frequently used have insufficient speeds of moisture release, sheet products are not fully regenerated, and their moisture absorption capability at an initial stage is sometimes not maintained. They have therefore problem that the time period for moisture release is increased or that an air conditioning apparatus or a total heat exchanger device needs to be increased in size for improving the heating capability for moisture release.
For overcoming these problems, it has been desired to develop a moisture adsorbent that is improved in moisture absorption amount, moisture absorption speed and moisture release speed.
Meanwhile, a sheet product containing a moisture adsorbent is in many cases required to have heat resistance, and hence many sheet products using inorganic fibers are used. As a method for the production thereof, there has been proposed a method in which an inorganic fiber paper is shaped in the form of a honeycomb and it is then calcined at a high temperature to remove an organic substance, followed by impregnation with an application liquid containing a moisture adsorbent and then drying at a high temperature (JP6-226037A), a method in which a ceramic fiber paper is impregnated with water glass to generate silica gel (JP5-115737A), or the like. These sheet products using inorganic fibers have had a problem that since they are hard and fragile, they are poor in impact resistance or susceptible to a large amount of moisture adsorbent powder falling off. In sheet products using inorganic fibers, further, high-temperature calcining is carried out for decreasing a weight, so that not any organic moisture adsorbent can be used. Further, the limitation imposed on the selection of a moisture adsorbent is that even inorganic moisture adsorbents cannot be used if they suffer a change in physical properties such as crystal structure, etc., at a high temperature.
For overcoming the impact resistance and the limitation imposed on the selection of a moisture adsorbent, sheet products comprising a moisture adsorbent and an organic fiber have been proposed. For example, there have been proposed a paper for a total heat-exchanger, which comprises a moisture adsorbent, a fiber for paper making and a thermally fusible substance (JP10-212691A), a paper for a total heat-exchanger, which comprises a moisture adsorbent, a fiber for paper making and cellulose that is converted to microfibril (JP11-189999A), a humidity-adjusting sheet comprising a cellulose fiber and a moisture adsorbent (JP2004-68188A), a substrate comprising a moisture adsorbent and an organic fiber (US Patent Application Publication No. 2002/0070002), and an adsorption element comprising a flame-retarding synthetic pulp, a polyvinyl alcohol-containing binder and a moisture adsorbent (JP2004-268020A). Since these sheet products using organic fibers are broken due to an impact to less degree and involve no high-temperature calcining step, the limitation imposed on the selection of a moisture adsorbent is alleviated. However, there has not yet been completely overcome the problem of a moisture adsorbent powder falling off during the production of an article by corrugating a sheet product or winding a sheet product in the form of a rotor or during the use of a sheet product as a packing material or in an air-conditioning apparatus. In particular, when the content of a moisture adsorbent in a sheet product is increased up to 30 mass % or more for increasing the moisture absorption amount, the amount of a powder that falls off is large. When the content of a moisture adsorbent is increased for preventing a powder from falling off, it is required to increase the amount of a sheet product for attaining an intended relative humidity, which results in a problem of an increase of an air-conditioning apparatus or a total heat exchanger device in size.